In many applications, it is desirable and/or necessary that a "basic" circuit be capable of being switched from one mode of operation to another. For example, it is desirable that the basic amplifying section of a circuit formed on an integrated circuit, such as a microprocessor chip, be operable either as an astable multivibrator oscillator, when connected to an RC network, or as an LC, RLC, crystal or tuning fork oscillator, when connected to an LC or RLC network or a crystal or tuning fork (typically located off-chip).
A problem exists in that certain circuit requirements needed to provide good and stable operation of the oscillator circuit are inconsistent with other requirements of the circuit. For example, in the amplifying section of a crystal oscillator circuit operated at frequencies below 30-40 KHz, the rise and fall times of the clock signal transitions are relatively slow. The slow rise and fall times cause the various amplifying stages (e.g. inverters) within the amplifying section of the oscillator circuit to be operated in their high gain region for a relatively long portion of each cycle. As a result, there is a greater likelihood of noise spikes and switching transients being amplified and causing disturbances of the clocking signal. Also, the slow rise and fall times cause a relatively high power dissipation. The rising and falling edges of the signal may be sharpened by using a positive (regenerative) feedback network within the amplifying section such that the amplifying section as a whole or certain stages within the amplifying section exhibit hysteresis, or Schmitt-trigger type action. The switching point of a stage exhibiting hysteresis (Schmitt-trigger action) depends on whether the input signal to the stage is increasing or decreasing and, typically, is significantly higher in one direction and lower in the other than the midpoint of the power supply voltage. However, a higher power supply voltage must be applied to the amplifying section of a crystal oscillator circuit to initiate oscillation when the amplifying section includes Schmitt-trigger type circuitry. A "higher" power supply voltage is not always available, and, where available, it causes an increase in the power dissipation of the system, which is highly undesirable. However, as noted above, "hysteresis" is needed for proper operation.
Furthermore, hysteresis circuits are needed in astable multivibrator circuits. For example, where the frequency determining element is an RC type network, it is essential to have Schmitt-trigger type circuitry to control the frequency of oscillation.
Thus, a problem exists where the same basic oscillator amplifying circuit is intended to be interconnected with crystals designed to oscillate at frequencies ranging from less than 30 to 40 KHz to more than 2 to 3 MHz. The problem is further aggravated where the same amplifying circuit is to be used to form an oscillator with elements such as a tuning fork or an RC, LC or RLC type network.